The present invention relates to the process of hot rolling of ceramics. More specifically, this invention involves the hot rolling of ceramics in conjunction with the self propagation synthesis process.
There has developed a great need for high technology ceramics such as TiB.sub.2 or B.sub.4 C in a variety of applications where extreme strength for a given weight of material is required. The customary process for producing such ceramics is by the batch process of hot pressing or sintering. Hot pressing typicaly in graphite dies, affords the highest density and finest grain size for good physical properties of the ceramics. However, hot pressing is a time consuming process limited by the size and number of dies that are available and especially the time-temperature-pressure constraints. A possible approach to resolving this issue would be to hot roll ceramics. However, ceramics typically do not show sufficient plasticity to be hot rolled in bulk form nor sufficiently high rates of densification if hot rolled in powder form to allow this to be done at practical pressures, temperatures and rates.
Recently there has been substantial interest in the processing of ceramics by self-propagating synthesis (SPS), otherwise known as self-propagating high temperature synthesis (SPHTS) or SHATS. This method involves the use of a chemical reaction to form the desired compound. A typical example and one of considerable interest, is the formation of TiB.sub.2 by the mixing of titanium and boron powders uniformly and then consolidating them in a pellet. The reaction is then ignited in the pellet, for example by heating one end of the pellet by a small electrical coil to the point where the reaction between the elemental powder constituents becomes spontaneous and the reaction propagates through the body. While this is potentially an intriguing method of processing, it has a variety of serious questions. One is whether it is really significantly more energy efficient than conventional processing. Another intrinsic problem is the fact that it can involve substantial amounts of porosity. First, there are, of course, pores between the powder particles that must be eliminated. Secondly, the reaction to form the compounds intrinsically generates porosity since the density of the resultant compounds is invariably greater than the average density of the starting constituents. Note that because of these porosity issues, it has commonly been either necessary or important to apply a mechanical load to the compact during reaction in order to consolidate it to high density, further reducing some of the possible advantage of this type of processing over hot pressing.